Emission of Microplastics via Sea Spray Aerosol: A Generalizable Enrichment Framework for Marine Constituents

Micro- and nanoplastics (MNPs) are emerging atmospheric pollutants, yet the processes controlling their transfer from the ocean to the atmosphere remain poorly constrained. Sea spray aerosol (SSA), produced when bubbles generated by breaking waves burst at the ocean surface, provides a plausible pathway for transferring MNPs and other marine constituents from seawater to air. Given their potential for inhalation exposure, long-range atmospheric transport, and deposition into remote ecosystems, constraining oceanic MNP emissions is necessary for evaluating their environmental fate, exposure pathways, and potential public-health risks. However, current understanding of this transfer is limited by uncertainty in how MNP enrichment in emitted droplets depends on particle properties, droplet production pathways, and environmental parameters. This dissertation addresses these gaps by combining controlled laboratory experiments with a pathway-resolved framework for representing oceanic emissions of MNPs and other preferentially enriched marine aerosol constituents in atmospheric models. First, MNP aerosolization was quantified in a Marine Aerosol Reference Tank (MART) using MNPs of different sizes, densities, and concentrations in synthetic seawater. These experiments showed that MNPs with diameters up to $10 mu m$ can be emitted via bubble bursting, that aerosolization increases with particle concentration in water and decreases with increasing particle size, and that particle density and water-column distribution influence transfer efficiency. The resulting experimentally based parameterization provided an initial constraint on global oceanic MNP emissions. Second, this dissertation addressed the need for droplet pathway separation in sea spray source functions, which are mathematical representations used in atmospheric models to estimate size-resolved SSA emission. When bubbles burst at the water surface, they can produce film drops through fragmentation of the bubble cap and jet drops through collapse of the bubble cavity. Because film and jet drops differ in their production mechanisms, size ranges, and enrichment behavior, treating SSA production as a single bulk size-resolved flux can be insufficient for marine constituents whose transfer depends on droplet production pathway. A Spray Aerosol Pathway Tank (SAPT) was therefore used to generate distinct bubble populations associated with film- and jet-drop-dominated aerosol production and to develop a film- and jet-drop-resolved sea spray source function. This source function represents SSA production as separate film and jet components and enables pathway-specific enrichment factors to be coupled with pathway- and size-resolved aerosol fluxes. The resulting pathway-resolved source function curve was broadly consistent with the range of widely used sea spray source functions, while additionally resolving the film and jet drop contributions needed to represent pathway-dependent enrichment. Demonstration calculations for the emission flux of dissolved organic carbon, calcium, saccharides, bacteria, MNPs, and per- and polyfluoroalkyl substances showed that pathway resolution can substantially alter the magnitude and size distribution of predicted marine constituent emissions. The dissertation then examined how MNP surface and environmental properties influence pathway-resolved transfer. Experiments with spherical $1 mu m$ surface-modified polystyrene particles showed that hydrophobic particles were enriched in jet drops by approximately one order of magnitude more than hydrophilic particles, whereas no clear wettability dependence was observed for film drops, suggesting that MNP particles may have complex effects on bubble film stability, bursting, and enrichment dynamics. Finally, experiments with environmentally representative MNPs showed that mechanically fragmented polylactic acid particles were not aerosolized as a random subset of the water population; instead, film and jet drops preferentially transferred smaller and more compact fragments. Experiments with pristine, mechanically abraded, and biofouled $8 mu m$ polystyrene particles further showed that enrichment increased after mechanical abrasion and increased further after biofouling. Overall, this dissertation demonstrates that oceanic bubble bursting can transfer MNPs from seawater to air and that this transfer depends on particle size, concentration, density, wettability, morphology, biofouling state, and droplet production pathway. By linking laboratory measurements of pathway-separated MNP aerosolization with a film- and jet-drop-resolved SSA source function, this work provides a physically informed framework for improving oceanic MNP emission estimates and for representing other preferentially enriched marine aerosol constituents in atmospheric models.